This invention relates to the recording sheet conveying mechanism of a thermal printer.
FIGS. 8 and 9 are a perspective view and a side view of the recording sheet conveying mechanism in a conventional thermal printer. Recording sheets 30 are supplied from a sheet supplying mechanism 15 one at a time. The front edge portion of each sheet 30 thus supplied is inserted into a clamper 10. Under this condition, the clamper 10 is closed by means of a clamper closing mechanism 16, so that the recording sheet is held by the clamper. The clamper 10 is mounted, on a bridge 10a both ends of which are mounted on a pair of endless timing belts 3 in such a manner that the clamper is in parallel with a platen roller 1. A first pair of pulleys 2 are mounted on the shaft of the platen roller 1 in such a manner that they are rotatable around the shaft of the platen roller. A second pair of pulleys 4 are driven through a torque limiter 13 by a second motor 12. Hence, the timing belts 3 are driven by the second pair of pulleys 4, and the clamper 10 is moved in the direction of the arrow B as the timing belts are driven. The speed of movement of the clamper 10 is determined from the number of revolutions per unitary time N.sub.2 of the second pair of pulleys 4 (hereinafter referred to as "the speed of rotation or merely speed N.sub.2 ", when applicable). The speed N.sub.2 of the second pulleys 4 has been determined from the constant speed M of the second motor 12 unless the torque limiter suffers from slip.
The clamper 10 thus moved is returned to its initial position passing through the first pair of pulleys 2, the second pair of pulleys 4 and a third pair of pulleys 5. In this operation, the recording sheet held by the clamper 10 is pushed against the platen roller 1 by a thermal head 9 so that the color of an ink sheet adapted to supply a printing color agent is transferred onto the recording sheet. In a color printing operation, the above-described color transferring operation is repeated three or four times using ink sheets different in color. In this case, the recording sheet is caused to move over the pulleys 2, 4 and 5 three or four times.
In the color transferring operation; i.e., in the printing operation, the nip region of the thermal head 9 which is in contact with the recording sheet 30 is pushed against the platen roller 1 by a force P.sub.H (cf. FIG. 3). Hence, the recording sheet 30 is conveyed as the platen roller 1 is rotated by a first motor 11; that is, the recording sheet 30 is conveyed at a constant speed V.sub.1 which is determined from the speed of the platen roller 1, and accordingly the clamper 10 holding the recording sheet is also moved at the same speed V.sub.1.
During printing, the speed of movement of the recording sheet or clamper is V.sub.1, as was described above. When the recording sheet 30 is not printed; i.e., when it is not pushed by the nip region of the platen roller through the nip region (hereinafter referred to as "an idle movement period", when applicable), the speed of movement of the clamper is V.sub.2.
The speed of movement V.sub.2 is set to a value higher than the recording sheet conveyance speed V.sub.1. The difference between the two speeds V.sub.1 and V.sub.2 is absorbed by the slip of the torque limiter 13. That is, for the printing period, the speed of movement of the clamper 10 and the timing belts 3 is equal to the sheet conveying speed V.sub.1, and the second pair of pulleys are rotated at the speed of rotation N.sub.1 corresponding to the sheet conveyance speed V.sub.1. The speed of rotation N.sub.1 of the second pulleys is lower than that N.sub.2 in the idle movement period in which the recording sheet is not printed. Therefore, the difference between the speed of rotation of the second pair of motor to rotate the second pulleys at the speed of rotation N.sub.2 at all times and the actual speed of rotation N.sub.1 of the second pulleys is absorbed by the slip of the torque limiter 13. In this slip, the torque predetermined by the torque limiter 13 is applied through the second pair of pulleys 4 and the timing belts 3 and 3 to the clamper 10. Accordingly, during printing, the clamper 10 draws the recording sheet 30 with a tensile force corresponding to the predetermined torque.
If the recording sheet conveyance speed changes during printing, then in the case of a color printing operation an undesirable color shifting phenomenon occurs. Hence, it is one of the most important conditions for a thermal printer to maintain the recording sheet conveyance speed during printing. During printing, the recording sheet conveyance speed depends on the nip region X of the thermal head 9 and the platen roller 1.
In the conventional thermal printer constructed as described above, the tensile force acting on the recording sheet may change because of the following reasons:
The output torque of the torque limiter 13 changes, which applies the tensile force to the recording sheet 30. This change is unavoidable because it is due to the operating principle of the torque limiter 13. At the nip region X, both the recording sheet 30 and an ink sheet 6 are conveyed. Thereafter, the recording sheet and the ink sheet are separated from each other. The force of separating the recording sheet 30 and the ink sheet from each other depends on the printing density of the thermal head; that is, the tensile force applied to the recording sheet is affected thereby.
The width of variation of the tensile force is, in general, smaller than the frictional force acting on the recording sheet at the nip region X (that is, the width of variation of the tensile force is not so large as to allow the recording sheet to slip through the nip region X. However, it should be noted that, for instance in the case where the tensile force increases in the direction of conveyance of the recording sheet, the recording sheet conveyance speed at the nip region increases with the variation of the tensile force. In the case where, on the other hand, the tensile force decreases in the direction of conveyance, the recording sheet conveyance speed at the nip region X decreases. In short, slip occurs between the recording sheet 30 and the nip region X of the platen roller, to change the speed in the above-described manner. This fact has been theoretically proven as indicated in FIG. 3 of the publication "The Rolling Contacts of Two Elastic-Layer-Covered Cylinders Driving a Loaded Sheet in the Nip" T.-C. Soong et al Transactions of the ASME (American Society of Mechanical Engineers) Journal of Applied Mechanics December, 1981, vol 48, pp 889-894 (hereinafter referred to as "Literature 1".
As was described above, in the conventional thermal printer, the speed of conveyance of a recording sheet 30 changes, even though it should be maintained constant.